linux-kernel - Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <xoigewp763crp6r5nxbrvlwjjcnhivbkfmyx4vc3kcjq63pzgr@znwfmvny6mha>
Date: Fri, 16 May 2025 12:17:05 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Shivank Garg <shivankg@....com>, Ard Biesheuvel <ardb+git@...gle.com>, 
	linux-kernel@...r.kernel.org, x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Ingo Molnar <mingo@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Brian Gerst <brgerst@...il.com>, "Rao, Bharata Bhasker" <bharata@....com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

On Thu, May 15, 2025 at 09:11:31PM +0200, Borislav Petkov wrote:
> On Thu, May 15, 2025 at 11:50:17PM +0530, Shivank Garg wrote:
> > I've re-tested the performance concerns we discussed earlier regarding 5-level paging.
> > Recent tests on a current kernel don't show any performance issues:
> > 
> > AMD EPYC Zen 5 (SMT enabled).
> > Linux HEAD 6.15.0-rc6+ 088d13246a46
> > 
> > lmbench/lat_pagefault:
> > numactl --membind=1 --cpunodebind=1 bin/x86_64-linux-gnu/lat_pagefault -N 100 1GB_randomfile
> > 
> > Output values (50 runs, Mean, 2.5 percentile and 97.5 percentile, in microseconds):
> > 
> > 4-level (no5lvl option)
> > Mean: 0.138876
> >      2.5%     97.5%
> > 0.1384988 0.1392532
> > 
> > 4-level (CONFIG_X86_5LEVEL=n)
> > Mean: 0.137958
> >      2.5%     97.5%
> > 0.1376473 0.1382687
> > 
> > 5-level
> > Mean: 0.138904
> >      2.5%     97.5%
> > 0.1384789 0.1393291
> > 
> > After repeating the experiments a few times, the observed difference(~1%) in mean values
> > is under noise levels.
> > I think these results address the performance concerns previously raised[1]. I don't
> > foresee any issues in proceeding with the 5-level paging implementation
> > simplification efforts[2].
> > 
> > [1] https://lore.kernel.org/all/80734605-1926-4ac7-9c63-006fe3ea6b6a@amd.com
> > [2] https://lore.kernel.org/all/20240621164406.256314-1-kirill.shutemov@linux.intel.com
> 
> I guess Kirill could dust off his patchset from [2] and that would get rid of
> CONFIG_X86_5LEVEL and likely simplify that aspect considerably...

https://lore.kernel.org/all/20250516091534.3414310-1-kirill.shutemov@linux.intel.com/

-- 
  Kiryl Shutsemau / Kirill A. Shutemov